The two grammatical situations that need a comma before ‘and’

In response to my column last week, “No, we shouldn’t ever stop learning English grammar,” reader Gerson Palomo posted this question on punctuation usage: “What I learned from school is that the comma (,) should not be placed before the word ‘and.’ Am I right?”

I replied to Gerson that it depends on the usage and the intended sense. For instance, there’s no need for a comma before “and” in a two-item compound subject like this: “Tina and Larry bumped at each other at the park during a heavy downpour.” But a comma is perfectly in order in a compound sentence like this: “I arrived two hours late, and only then did I realize that I was mistaken about the time of our meeting.”

There are actually two specific situations that require placing a comma before “and,” a conjunction that we will recall joins sentence elements of the same grammatical rank or function. Those situations are as follows:

1. A comma is needed before “and” to punctuate the last of a series of three or more additive grammar elements, as in “Temperate countries have the four seasons of summer, autumn, winter, and spring.” For clarity, the so-called serial or Oxford comma is formally placed before the last of the grammar elements, which in this case is “spring.” However, this convention is not rigorously observed in modern usage, particularly in the case of serial grammar elements of one or just a few words. Indeed, most newspapers and magazines streamline such sentences by routinely knocking off that last comma: “Temperate countries have the four seasons of summer, autumn, winter and spring.”
When the serial grammar elements are long and complex, however, using the Oxford comma greatly ensures clarity. Its absence in this sentence construction from a magazine article is particularly instructive: “The major businesses in the domestic pet services industry are traditional veterinary services, fancy pet grooming and makeover shops, a wide assortment of animal and bird food, freshwater and marine fish of various kinds and aquarium equipment and supplies for industrial and home use.”

There are actually five serial grammar items in that sentence (try counting them mentally), but because the writer didn’t deploy an Oxford comma after the word “kinds” before adding the last serial item, the latter part of the sentence almost becomes incomprehensible. Now see how the simple insertion of the Oxford comma effortlessly clarifies the serial enumeration: “The major businesses in the domestic pet services industry are traditional veterinary services, fancy pet grooming and makeover shops, a wide assortment of animal and bird food, freshwater and marine fish of various kinds, and aquarium equipment and supplies for industrial and home use.”

2. A comma is needed before “and” to coordinate two independent clauses in a compound sentence, as in “The sudden surge in the survey ratings of an underrated presidential candidate caused panic among his opponents, and suddenly they all went on overdrive to demolish his reputation.” The comma between “opponents” and “and suddenly” makes it crystal clear that two independent clauses are involved, the first being “The sudden surge in the survey ratings of an underrated presidential candidate caused panic among his opponents” and the second, “suddenly they all went on overdrive to demolish his reputation.” Without that comma between those two clauses, the sense and clarity of the compound sentence is greatly diminished.

This rule to use a comma before the conjunction applies not only to “and” but also to the six other coordinating conjunctions “but, “or,” “nor,” “for,” “so,” and “yet.” For example, a comma is needed before “for” to clearly establish the sense of this compound sentence: “The sudden surge in the survey ratings of an underrated presidential candidate clearly panicked his opponents, for suddenly they all went on overdrive to demolish his reputation.”